This application claims the priority of Korean Patent Application No. 2003-49539, filed on Jul. 19, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a feedwater control system and a control method therefor, and more particularly, to a feedwater control system considering a pressure drop of a feedwater control valve in a nuclear power plant and a control method therefor, which are capable of constantly maintaining the pressure drop of the feedwater control valves regardless of a nuclear reactor power or kinds and number of feedwater equipments.
2. Description of the Related Art
A nuclear power plant generally includes systems having 100 or more separate functions. The systems include a nuclear steam supply system (NSSS) centering on a nuclear reactor, a turbine/generator system which receives steam to turn a generator to generate electricity, and other additional equipments. At present, a pressurized water reactor (PWR) is mainly used in Korean nuclear power plants and includes a primary system centering on a nuclear reactor, a secondary system having a steam generator, a turbine, a generator and a condenser, an engineering safety equipment system in preparation for accidents, a power transmission/ distribution system, a measurement/control system, and other auxiliary systems.
Hot water generated at the nuclear reactor is circulated through a heat transfer pipe of the steam generator, which is connected through a coolant pipe to the nuclear reactor. Also, the hot water transfers heat to water that is fed into the steam generator through another pipe, and then returns to the nuclear reactor. In order to perform such operations naturally, the steam generator must properly maintain the water level. This is usually achieved by a feedwater control system of the nuclear power plant that constantly controls the water level of the steam generator.
FIG. 1 illustrates electrical paths in a feedwater control system of a nuclear power plant according to the prior art. Referring to FIG. 1, the feedwater control system of a nuclear power plant includes a detection block 110 for generating a water level correction error signal 115c, and a control block 130 for controlling a main feedwater pump 103, a main feedwater control valve 105 and a downcorner control valve 107 based on a flow request signal 131c, which is processed through a proportional integrator 131.
The detection block 110 detects a water level correction error and includes a flow error signal generator 111 for generating a flow error signal 111c, and a water level correction error signal generator 115 for generating a water level correction error signal 115c. The flow error signal generator 111 receives a steam flow signal 111 a corresponding to a steam flow, which is discharged from a steam generator, and a feedwater flow signal 111b corresponding to a feedwater flow, which is introduced into the steam generator, and generates the flow error signal 111c. The flow error signal 111c is input to the water level correction error signal generator 115. The water level correction error signal generator 115 sums the flow error signal 111c and a water level measurement signal and generates the water level correction error signal 115c. Here, the water level measurement signal is a signal corresponding to a difference between a water level measurement signal 115a and a water level setup signal 115b. The water level measurement signal 115a corresponds to a measured water level of the steam generator and the water level setup signal 115b corresponds to a previously set water level of the steam generator.
The control block 120 passes the water level correction error signal 115c through the proportional integrator 131, thereby generating the flow request signal 131c. Then, a main feedwater pump speed signal converter 133, a main feedwater control valve opening signal converter 138 and a downcorner control valve opening signal generator 139 convert the flow request signal 131c into control signals for controlling a main feedwater pump 103, a main feedwater control valve 105 and a downcorner feedwater control valve 107, respectively. The main feedwater pump 103, the main feedwater control valve 105 and the downcorner feedwater control valve 107 are controlled by the control signals. As a result, the water level of the steam generator is adjusted. Since the main feedwater pump 103 is connected to two steam generators through a common pipe, the flow request signal 131c from the proportional integrator 131 to the main feedwater pump 103 passes through a high signal selector 132 before it is input to the main feedwater pump speed signal converter 133. The flow request signal 131c is compared with another flow request signal output from a feedwater control system of another steam generator. A high flow request signal is selected and output to the main feedwater pump speed signal converter 133.
The above-mentioned method is a three-factor control method of generating the flow request signal, which is a control signal, using three variables as a control input signal in a high power control mode of about 20% or more nuclear reactor power. The three variables are the water level measurement signal, the feedwater flow signal and the steam flow signal. However, a one-factor control method is used in a low power mode of 20% or less nuclear reactor power. In other words, the flow request signal is generated using only the water level measurement signal because the feedwater flow signal and the steam flow signal have a low reliability.
Kinds and number of feedwater equipments used in the nuclear power plant depend on the generated nuclear power. In the low power mode of 20% or less nuclear reactor power, the main feedwater control valve is closed and the main feedwater pump operates at a lowest speed. Therefore, an actual feedwater flow control function is performed by the downcorner feedwater control valve. In the high power mode of 20% or more nuclear reactor power, the downcorner feedwater control valve is opened to pass about 10% of a whole feedwater flow and the feedwater flow control function is performed by the main feedwater control valve 105 and the main feedwater pump 103. Also, in the 20% or more nuclear reactor power, only one main feedwater pump operates in 50% or less nuclear reactor power. Meanwhile, in the 50% or more nuclear reactor power, two main feedwater pumps operate. In the 100% nuclear reactor power, three main feedwater pumps may operate at the same time.
The feedwater control system of the steam generator according to the prior art does not consider pressure drop of the feedwater control valve. The pressure drop cannot be maintained constantly according to the nuclear reactor power or the kinds and number of the operating feedwater equipment.
In a low power operation section of the nuclear reactor, the main feedwater pump speed is maintained constantly. Therefore, while a front pressure of the downcorner feedwater control valve is constantly maintained, a rear pressure of the downcorner feedwater control valve is changed depending on a pressure variation of the steam generator. The steam generator operates at low or high pressures due to various factors, such as a nuclear reactor power control method or an operation way of operators. Accordingly, in the low power operation section, pressure drop of the downcorner feedwater control valve is greatly changed. Such change of the pressure drop has a bad effect in controlling the water level of the steam generator and can be determined from the following equation:                     Q        =                  Cv          ×                                                    Δ                ⁢                                                                   ⁢                P                            γ                                                          (        1        )            
where Q is a feedwater flow, Cv is a flow coefficient, γ is a specific weight, and ΔP is a pressure drop.
As can be seen from the equation (1), the feedwater flow passing through the valve is a function of the pressure drop, the flow efficient, and the specific weight. Among them, the specific weight is a function of the feedwater temperature and is a predictable value. The flow coefficient is a function of the opening of the downcorner feedwater control valve. The flow coefficient is reflected on the downcorner control valve opening signal converter and is a predictable value. Accordingly, it can be seen that the pressure drop of the downcorner feedwater control valve is a main factor to change the feedwater flow. However, in the low power operation section, the feedwater control system of the steam generator uses only the water level of the steam generator as an input. Thus, such flow change according to the pressure variation cannot be reflected rightly. If the pressure of the steam generator in the low power of the nuclear reactor is not stable, it is difficult to stably control the water level of the steam generator.
FIG. 2 illustrates a variation in pressure drop of the main feedwater control valve according to power of the main feedwater control valve in the conventional feedwater control system of the nuclear power plant. Referring to FIG. 2, in a high power operation section of the nuclear reactor, when one main feedwater pump is operating and another main feedwater pump is additionally operating since the power becomes 50% or more, a pressure drop applied to the main feedwater control valve is greatly increased. Such a phenomenon occurs due to characteristics of pressure head versus flow in the main feedwater pump. When one main feedwater pump is operated, a flow passing one main feedwater pump is decreased, such that the pressure head is increased. The increase of the feedwater flow and the water level rise of the steam generator due to the rapid rise of the pressure head makes the flow request signal decrease. Since the flow request signal controls the main feedwater pump speed and the opening of the main feedwater control valve at the same time, the opening of the main feedwater control valve as well as the main feedwater pump speed is decreased. Accordingly, since the main feedwater pump speed is not reduced as much as the increased pressure head, the front pressure of the main feedwater control valve rises, thereby increasing the pressure drop of the main feedwater control valve. Further, such phenomena occur severely in nuclear power plants in which three main feedwater pumps are operating at 100% power. If the pressure drop of the main feedwater control valve is changed according to the power, several problems may occur. In case when the pressure drop of the main feedwater control valve is high, the control may be unstable. On the other hand, in case when the pressure drop of the main feedwater control valve is low, the water level control capability is reduced, so that the control is difficult.
Considering these problems, the conventional nuclear power plants uses a method of measuring a pressure drop of the main feedwater control valve and controlling the speed of the feedwater pump. However, this method is constituted with an independent control system, asides from the water level control of the steam generator. Since the speed of the main feedwater pump is controlled using only the valve pressure drop as an input, it is difficult to properly cope with the rapid transition state in the water level of the steam generator. If the input signals are lost, the rapid transition state may occur, so that this method is not a proper approach.
In the conventional feedwater control system of the nuclear power plant and the control method therefor, the severe variation in the pressure drop of the feedwater control valve has a bad effect in controlling the water level of the steam generator. However, it is difficult to stably control the water level of the steam generator because the flow variation according to such pressure drop is not reflected rightly. Specifically, if the pressure drop of the main feedwater control valve is high, the control may become unstable. Also, if the pressure drop of the main feedwater control valve is low, the performance in the water level control of the steam generator is degraded. Accordingly, it is difficult to select the setup values of the feedwater control system in order to solve such problems. Also, it is difficult to optimize the control setup values.